The present invention relates to a fuel cells system that enables fuel cells to be activated with a high efficiency of energy conversion, as well as to a method of controlling such fuel cells.
As shown in an example of FIG. 5, in a background art fuel cells system mounted on an electric vehicle, a reformer unit 128 receives supplies of fuel 124, for example methanol and water, fed via a pump 126 and produces a hydrogen-containing gaseous fuel from the fuel 124 through a steam reforming reaction of methanol. Fuel cells 136 receive a flow of the produced gaseous fuel and the air 130 and generate an electromotive force through electrochemical reactions of the gaseous fuel and the air 130. The electric power generated by the fuel cells 136 and the electric power output from a battery 140, which is connected in parallel with the fuel cells 136, are supplied to an inverter 144 to drive a motor 146 and obtain a driving force of the electric vehicle.
A control unit 120 calculates a required output (required electric power) of the inverter 144 from an accelerator travel of the electric vehicle measured by an accelerator pedal position sensor 122, and regulates the inverter 144 based on the calculated required output. Such regulation causes electric power corresponding to the required output to be supplied to the motor 146 via the inverter 144.
The fuel cells 136 output the electric power to cover the required output of the inverter 144. When the electric power output from the fuel cells 136 is insufficient for the required output, the battery 140 outputs the electric power to the inverter 144 to compensate for the insufficiency. The output electric power of the fuel cells 136 accordingly depends upon the required output of the inverter 144.
In response to a requirement of the output of electric power from the inverter 144, the fuel cells 136 can not output the required electric power in the case in which the gaseous fuel supplied from the reformer unit 128 to the fuel cells 136 is not sufficient for the output of the required electric power. That is, the output electric power of the fuel cells 136 also depends upon the quantity of the gaseous fuel (that is, the gas flow rate) fed to the fuel cells 136.
The control unit 120 drives the pump 126 based on the required output of the inverter 144, and regulates the quantities of the fuel 124 fed to the reformer unit 128, in order to regulate the quantity of the gaseous fuel supplied to the fuel cells 136 according to the required output of the inverter 144.
The quantity of the gaseous fuel produced by the reformer unit 128 does not immediately increase (or decrease) with an increase (or a decrease) in supplied quantities of the fuel 124, but increases or decreases after a time lag of 2 to 20 seconds. The quantity of the gaseous fuel required for the fuel cells 136 is thus not always identical with the actual supply of the gaseous fuel (the gas flow rate) to the fuel cells 136.
As described above, in the background art fuel cells system, the output electric power of the fuel cells 136 depends upon the required output of the inverter 144 and upon the quantity of the gaseous fuel (the gas flow rate) supplied to the fuel cells 136. The working point of the fuel cells 136 is thus varied with variations in required output of the inverter 144 and in gas flow rate.
FIG. 6 is a characteristic chart showing variations in power generation efficiency versus the output electric power in general fuel cells with a variation in quantity of the gaseous fuel (the gas flow rate) supplied to the fuel cells as a parameter. FIG. 7 is a characteristic chart showing a variation in output electric power versus the required quantity of the gaseous fuel in general fuel cells.
In the background art fuel cells system described above, as shown in FIG. 6, although the fuel cells are capable of being activated at a working point xe2x80x9caxe2x80x9d of high power generation efficiency, the fuel cells may be activated, for example, at a working point xe2x80x9cbxe2x80x9d of low power generation efficiency since the actual working point is varied with a variation in gas flow rate.
In the background art fuel cells system described above, as shown in FIG. 7, even when a sufficient quantity Qc of the gaseous fuel is supplied from the reformer unit to the fuel cells to generate an output electric power Wc, the fuel cells may be activated, for example, at a working point xe2x80x9cdxe2x80x9d to generate only an output electric power Wd since the actual working point is varied with a variation in required output of the inverter. In this case, the quantity of the gaseous fuel required to generate the output electric power Wd is equal to only Qd, and the wasteful quantity of the gaseous fuel is (Qcxe2x88x92Qd). This lowers the utilization factor of the gaseous fuel.
As described above, in the background art fuel cells system, the working point of the fuel cells is varied with variations in required output of the inverter and in gas flow rate. The fuel cells are thus not always activated at the working point of high power generation efficiency or at the working point of high gas utilization factor.
The power generation efficiency and the gas utilization factor have a tradeoff relationship, so that it is difficult to enhance both the power generation efficiency and the gas utilization factor. Maximizing the product of the power generation efficiency and the gas utilization factor enhances both the power generation efficiency and the gas utilization factor as much as possible. The product of the power generation efficiency and the gas utilization factor is expressed as an energy conversion efficiency of the fuel cells.
An object of the present invention is thus to solve the problems of the background art and to provide a fuel cells system that enables fuel cells to have an enhanced energy conversion efficiency.
At least part of the above and the other related objects is attained by a first fuel cells system that has fuel cells, which receive a supply of a gas and generate electric power, and which supply the generated electric power to a load. The first fuel cells system includes: a gas flow rate-relating quantity measurement unit that measures a gas flow rate-relating quantity, which relates to a flow rate of the gas supplied to the fuel cells; and a control unit that specifies a working point associated with an output electric current-output voltage characteristic of the fuel cells corresponding to the observed gas flow rate-relating quantity, and regulates electric power to be taken out of the fuel cells, so as to cause the fuel cells to be activated at the specified working point.
The present invention is also directed to a first method of controlling fuel cells that receive a supply of a gas and generate electric power. The first method includes the steps of: (a) measuring a gas flow rate-relating quantity, which relates to a flow rate of the gas supplied to the fuel cells; (b) specifying a working point associated with an output electric current-output voltage characteristic of the fuel cells corresponding to the observed gas flow rate-relating quantity; and (c) regulating electric power to be taken out of the fuel cells, so as to cause the fuel cells to be activated at the specified working point.
The operation of the first fuel cells system and the corresponding first method of the present invention measures the gas flow rate-relating quantity, which relates to the flow rate of the gas supplied to the fuel cells, and specifies a working point associated with the output electric current-output voltage characteristic of the fuel cells corresponding to the observed gas flow rate-relating quantity. The operation then regulates the electric power to be taken out of the fuel cells, so as to cause the fuel cells to be activated at the specified working point.
In the first fuel cells system and the corresponding first method of the present invention, the working point of the highest energy conversion efficiency on the output electric current-output voltage characteristic is specified as the working point associated with the output electric current-output voltage characteristic corresponding to the observed gas flow rate-relating quantity. This arrangement enables the fuel cells to be activated at the working point of the highest energy conversion efficiency, thus enhancing both the power generation efficiency and the gas utilization factor of the fuel cells as much as possible.
The present invention is also directed to a second fuel cells system that has fuel cells, which receive a supply of a gas and generate electric power, and a secondary battery, which accumulates electric power therein and outputs the accumulated electric power. The second fuel cells system supplies at least one of the electric power generated by the fuel cells and the electric power output from the secondary battery to a load. The second fuel cells system includes: a gas flow rate-relating quantity measurement unit that measures a gas flow rate-relating quantity, which relates to a flow rate of the gas supplied to the fuel cells; and a control unit that specifies a working point associated with an output electric current-output voltage characteristic of the fuel cells corresponding to the observed gas flow rate-relating quantity, determines an amount of electric power to be taken out of the fuel cells, which is required to activate the fuel cells at the specified working point, as well as an amount of electric power to be supplied to the load, and regulates at least one of electric power to be output from the secondary battery and electric power to be accumulated in the secondary battery, based on the two amounts of electric power thus determined.
The present invention is also directed to a second method of controlling a secondary battery in a fuel cells system having fuel cells, which receive a supply of a gas and generate electric power, and the secondary battery, which accumulates electric power therein and outputs the accumulated electric power, and supplying at least one of the electric power generated by the fuel cells and the electric power output from the secondary battery to a load. The second method includes the steps of: (a) measuring a gas flow rate-relating quantity, which relates to a flow rate of the gas supplied to the fuel cells; (b) specifying a working point associated with an output electric current-output voltage characteristic of the fuel cells corresponding to the observed gas flow rate-relating quantity; (c) determining an amount of electric power to be taken out of the fuel cells, which is required to activate the fuel cells at the specified working point, as well as an amount of electric power to be supplied to the load; and (d) regulating at least one of electric power to be output from the secondary battery and electric power to be accumulated in the secondary battery, based on the two amounts of electric power thus determined.
The operation of the second fuel cells system and the corresponding second method of the present invention measures the gas flow rate-relating quantity, which relates to the flow rate of the gas supplied to the fuel cells, and specifies a working point associated with the output electric current-output voltage characteristic of the fuel cells corresponding to the observed gas flow rate-relating quantity. The operation subsequently determines the amount of electric power to be taken out of the fuel cells, which is required to activate the fuel cells at the specified working point, as well as the amount of electric power to be supplied to the load, and regulates the electric power to be output from the secondary battery or the electric power to be accumulated in the secondary battery, based on the two amounts of electric power thus determined. Regulating the electric power of the secondary battery in this manner causes the determined amount of electric power to be taken out of the fuel cells and enables the fuel cells to be activated at the specified working point.
In the second fuel cells system and the corresponding second method of the present invention, the working point of the highest energy conversion efficiency is specified as the working point associated with the output electric current-output voltage characteristic corresponding to the observed gas flow rate-relating quantity. This arrangement enables the fuel cells to be activated at the working point of the highest energy conversion efficiency through the regulation discussed above, thus enhancing both the power generation efficiency and the gas utilization factor of the fuel cells as much as possible.
In accordance with one preferable application of the present invention, the second fuel cells system further includes a state of charge sensor that measures a state of charge of the secondary battery. In this application, the control unit regulates at least one of the electric power to be output from the secondary battery and the electric power to be accumulated in the secondary battery, based on the observed state of charge in addition to the two amounts of electric power determined.
In a similar manner, it is preferable that the second method of the present invention further includes the step of (e) measuring a state of charge of the secondary battery. In this application, the step (d) includes the step of regulating at least one of the electric power to be output from the secondary battery and the electric power to be accumulated in the secondary battery, based on the observed state of charge in addition to the two amounts of electric power determined.
In general, the output electric power of the secondary battery depends upon the state of charge of the secondary battery. In the case in which the state of charge of the secondary battery is close to the full charge level, it is impossible to further accumulate the electric power in the secondary battery. The control is accordingly required to prevent the electric power from being further accumulated in such cases.
In either the first fuel cells system or the second fuel cells system of the present invention, it is preferable that the control unit specify a point of highest energy conversion efficiency on the output electric current-output voltage characteristic as the working point.
In a similar manner, in either the first method or the second method of the present invention, it is preferable that the step (b) includes the step of specifying a point of highest energy conversion efficiency on the output electric current-output voltage characteristic as the working point.
Specifying the working point in this manner enables the fuel cells to be activated at the working point of the highest energy conversion efficiency.
The present invention is also directed to a third fuel cells system, which includes: fuel cells that receive a supply of a gaseous fuel and an oxidizing gas and generate electric power through an electrochemical reaction of the gaseous fuel and the oxidizing gas; a flow sensor that measures a flow rate of at least one of the gaseous fuel and the oxidizing gas supplied to the fuel cells; a secondary battery that accumulates electric power therein and outputs the accumulated electric power; a state of charge sensor that measures a state of charge of the secondary battery; an inverter that receives a supply of electric power from at least one of the fuel cells and the secondary battery to drive a motor; a converter that varies a voltage output from the fuel cells and applies the varied voltage in parallel to the secondary battery and the inverter; and a control unit that specifies a working point associated with an output electric current-output voltage characteristic of the fuel cells corresponding to the observed flow rate, determines an amount of electric power to be taken out of the fuel cells, which is required to activate the fuel cells at the specified working point, determines an amount of electric power to be supplied to the inverter based on external information, and regulates the voltage output from the converter, based on the two amounts of electric power thus determined and the observed state of charge.
In the third fuel cells system of the present invention, the flow sensor measures the flow rate of at least the gaseous fuel or the oxidizing gas supplied to the fuel cells. The state of charge sensor measures the state of charge of the secondary battery. The inverter receives a supply of electric power from at least one of the fuel cells and the secondary battery to drive the motor. The converter increases or decreases the voltage output from the fuel cells and applies the varied voltage in parallel to the secondary battery and the inverter. The control unit specifies a working point associated with the output electric current-output voltage characteristic of the fuel cells corresponding to the flow rate observed by the flow sensor, and determines the amount of electric power to be taken out of the fuel cells, which is required to activate the fuel cells at the specified working point. The control unit also determines the amount of electric power to be supplied to the inverter based on external information. The control unit then regulates the voltage output from the converter, based on the two amounts of electric power thus determined and the state of charge observed by the state of charge sensor. This arrangement regulates the electric power of the secondary battery (either the output electric power or the accumulated electric power), to which the regulated voltage is applied, to a desired level. Such regulation causes the determined amount of electric power to be taken out of the fuel cells and enables the fuel cells to be activated at the specified working point.
In the third fuel cells system of the present invention, the working point of the highest energy conversion efficiency is specified as the working point associated with the output electric current-output voltage characteristic corresponding to the observed gas flow rate. This arrangement enables the fuel cells to be activated at the working point of the highest energy conversion efficiency through the regulation discussed above, thus enhancing both the power generation efficiency and the gas utilization factor of the fuel cells as much as possible.
The technique of the present invention may also be attained by an electric vehicle having any one of the first through the third fuel cells system mounted thereon. The electric vehicle has a motor as the load, which receives a supply of electric power from the fuel cells and is driven to give the driving force of the electric vehicle.
Mounting any of the first through the third fuel cells systems on the electric vehicle enhances the energy conversion efficiency of the electric vehicle.